1. Field of the Invention
The present invention relates to a technique for reducing the warpage created in dielectrically isolated (DI) wafers during the wafer thinning process by introducing a highly-strained layer near the back surface of the polysilicon support structure B of the DI wafer.
2. Description of the Prior Art
At one stage in the processing of dielectrically isolated (DI) wafers, the single crystal (100) silicon substrate must be thinned by a process which utilizes conventional grinding techniques. The grinding of the silicon substrate results in thinning the supporting single crystal part of the composite structure such that the subsequent removal of a portion of the single crystal results in wafer deformation of approximately 0.010".+-.0.001", where the positive sign of deformation indicates concave warpage of the thick polysilicon layer viewed from the polysilicon side which covers the silicon substrate. The drastic increase in wafer warpage during the grinding process, often referred to as springback, is undesirable as it places undesirable constraints on the ability to subject the wafers to automatic handling during subsequent IC processing, and may also cause irreversible structural damage in the single crystal tubs.
In order to overcome this springback problem, it was recognized that the creation of a subsurface damaged layer in the polysilicon would mitigate the affects of the springback problem during the substrate thinning process. In one method, subsurface damage can be created by locally melting the top 0.006"-0.008" of the polysilicon layer by utilizing lasers, electron beams, scanned strip heaters, or extended radiative heat sources (e.g., tungsten halogen lamps), followed by rapid solidification of the melted layer. These rapid thermal processes, as described in Laser and Electron Beam Interactions with Solids, (North-Holland, 1982) B. R. Appleton et al, Eds., and further described in Laser-Solid Interactions and Transient Thermal Processing of Materials (North-Holland, 1983), J. Narayan et al, Eds, function to create a structurally different polysilicon layer including a distinct interface which is capable of lessening the degree of springback.
An alternative technique, referred to as SIMOX (Separation by IMplanted OXygen), uses a buried SiO.sub.2 layer which is formed by a high dose (10.sup.18 atoms cm.sup.-2) deep oxygen-ion bombardment of the polysilicon layer. This technique is described in detail in the article "Formation of SiO.sub.2 Films by Oxygen-Ion Bombardment", by M. Watanabe et al, appearing in Japanese Journal of Applied Physics, Vol. 5, 1966, at pp. 737-738. As described in the article by Watanabe et al, SIMOX can be utilized in this case immediately prior to or after the final polish operation of the (100) silicon substrate to create a polysilicon subsurface disturbed layer. A further discussion of the SIMOX process can be found in the article "C.M.O.S. Devices Fabricated on Buried SiO.sub.2 Layers Formed by Oxygen Implantation Into Silicon", by K. Izumi et al, appearing in Electronics Letters, Vol. 14, No. 18, Aug. 31, 1978, at pp. 593-594.
Another alternative technique utilizing partial amorphorization of the top polysilicon surface prior to final polishing by a channeled silicon ion implant may also be utilized to create a subsurface damaged layer capable of reducing springback. Reference to this procedure may be found in the article "Disorder Produced by High-Dose Implantation in Si.sup.+ ", by L. Csepregi et al, appearing in Applied Physics Letters, Vol. 29, No. 10, Nov. 15, 1976, pp. 645-648.
A problem with all of the above-described processes, however, is that the polysilicon deposition must be completed before any of these techniques may be used. Additionally, all of these methods are complex, high temperature procedures which are rather expensive and not easily amenable to production. Furthermore, the disturbed layer formed by the above-described processes will not be able to generate a sufficient opposing force against the severe warpage springback which occurs in DI substrates. Therefore, a need remains in the prior art to provide a method for reducing springback which does not require significant additional processing and, ideally, may be directly incorporated into the polysilicon deposition process.